Space or play between a threaded nut and a threaded screw is generally referred to as backlash. Nut and screw assemblies may have both radial and axial backlash between the threads for various reasons, such as for example, limitations in manufacturing. Further, additional backlash can be generated during the life of the assembly from wear caused by friction. In application of the assembly, backlash can cause many problems, such as for example, inefficiencies and positioning errors.
To compensate for or reduce the effects of backlash, various prior art mechanisms have been developed. For example, two-piece threaded nuts for use with a threaded power screw are known in the art. One known strategy to eliminate axial and radial backlash in a two-piece nut assembly, while also compensating for the material wear, is to maintain a continuous internal force between two threaded nuts. A compression spring forcing two threaded nuts away from each other, creating a preload, has been used to eliminate backlash. Although the arrangement is both relatively easy to assemble and economical, the design presents several problems.
The force which the compression spring puts onto the opposing nuts is typically relatively large. A large amount of axial force creates a significant drag torque. The mechanical device which is driving the power screw must overcome this torque in operation. However, if the axial force is not relatively large, which would lower the drag torque, the load applied to the nut during its application may overcome the spring force. When the force of the compression spring is less than the force of the applied load, the preload becomes ineffective and the backlash is reintroduced.